Lecture 9: Voluntary Movement PDF

Lecture 9: Voluntary Movement ● ● Remember: in sensory we have primary sensory cortex, secondary unimodal & multimodal, while in motor we have primary motor cortex & the premotor cortex. Primary motor cortex (A4 or M1) Cerebral cortex and motor map: (old) ● Different parts of the body are represen...

Lecture 9: Voluntary Movement ● ● Remember: in sensory we have primary sensory cortex, secondary unimodal & multimodal, while in motor we have primary motor cortex & the premotor cortex. Primary motor cortex (A4 or M1) Cerebral cortex and motor map: (old) ● Different parts of the body are represented in the primary motor cortex (homunculus): ○ Motor outputs to lower parts → medial ○ Upper limbs → lateral ○ Fingers, face & mouth → represented by large areas, reflecting ↑ number of neurons needed for fine motor control Internal organisation of the motor map of the arm in the motor cortex (more elaborate): ● Inner central core → neurons that control the distal arm (digits & wrist) ● Surrounding intermediate core is an overlap area called proximal-distal co facilitation ● Outer core → neurons that control the proximal arm (elbow & shoulder) ● Each muscle can be activated by stimulating many widely dispersed sites A reaching movement is coded by a population of neurons in the arm motor map: ● Each neuron fires at a different intensity for movement in a particular direction ● The motor command for a movement is generated by a widely distributed population of neurons throughout the arm motor map. ● Cells with similar preferred directions are located at several different sites in the arm motor map & nearby cells often have different preferred directions. ● Let's take an experiment as an example: ○ A monkey had electrodes that were connected to his motor area & it was asked to move a handle either to the front, right, left & obliquely, in about 6 different directions ○ They were recording the activity of such movements in the nucleus when moving in different directions. ○ This is a recording of one of those neurons (green circle in previous slide picture B) and it was recorded 5 times. ○ They started with 600 neurons but only 200 were responsive to that specific direction of movement. ○ We can see that a single neuron will fire more for a specific direction than it would for another direction, so it will have different black lines that go in different locations but there are prominent ones that indicate the preference of that neuron, the size of each line represents the intensity of the activity. ○ Adding those different lines (vector) will give us a single vector that represents the direction of that neuron (red arrow) ● There are neurons in the motor cortex that allows us through their activity & their preference for the direction to know the monkey’s intention before he actually carries out the movement, I can predict where the monkey will reach ● This tell us that not the muscle that dictates the movement, it is the representation by the UMN ● There is an intention (in motor cortex) & an action (carried out by muscles) that come in sequence. Only a few millisec separate them Motor commands are population codes: ● Neurons that control the movement of individual fingers are distributed throughout the hand-control area of the primary motor cortex The premotor area (A6) is divided into: ● Lateral premotor area (for simple movements we don't need multiple plans to choose from) ● Supplementary motor area (we add this if complex, so we use lateral then add this) Function: ● Lateral premotor area & basal ganglia → involved in selection of movements to external cues (i.e. Visual, auditory sensory stimuli, etc) ● When the subject makes complex finger movement sequence, activity is seen in the finger area of the: ○ Primary motor cortex ○ Primary somatosensory cortex ○ Supplementary motor area ● When the subject imagines the same complex movement sequence, the activity is seen only in the supplementary motor area (in both side are activated together) ○ Supplementary activity seen when making a sequence with either hands (primary motor activity is always contra-lateral) ○ This bilateral activity is useful in movements that require coordination of both hands (e.g. tying shoe laces) ● Example: ○ If you need to carry out a complex sequenced movement, like touching your fingers with your thumb, one after the other, this very well arranged, harmonised, and complex movement will activate neurons in the primary motor cortex, sensory cortex (feeling the touch of your thumb), premotor cortex (because I have a specific plan of movement), & in the supplementary motor cortex (because it is a complex sequence of movement). ○ In this case you are thinking of the movement and carrying it out, but if you were only to think of the movement without actually doing it you will only have activity in the supplementary motor cortex ■ This means that the supplementary motor cortex is activated by both external (lateral only) & internal stimuli ■ The lateral motor cortex is only activated by external stimuli ● Clinical stimulation: ○ Motor cortex → pt move specific parts (fingers for example) in contralateral side. This tells us that the motor cortex → “simple movements of different muscles” ○ Supplementary motor cortex → “complex sequence of movement, it doesn’t have a specific area for a specific muscle” → bilateral activation of those movements ■ This is extremely important for complex movements like buttoning shirt you need to have a sequence of movements between both your right & left hand to ensure that the button gets in the hole. Supplementary motor area (SMA): ● Involved in planning, generation & control of sequential motor actions. ● Can be activated by both external & internal cues (i.e. memory) when the selection of movement sequences is required. ● Active before & during voluntary movements ● Stimulation can evoke movements on both sides of the body or halt ongoing voluntary movements. ○ Suggesting that this area has a role in coordinating movements on the two sides of the body. Premotor area: ● Has a motor map similar to that of M1 (more intensity is needed) → same map in the primary motor area. ● Projects to primary cortex & SC (can initiate movement WITHOUT primary motor area) ● The existence of these direct mono-synaptic connections suggests that the premotor neurons can control hand movements independently of the primary motor cortex. ● Gives multiple possible plans for the movement you want to make ○ The basal ganglia will choose the plan Sensory-motor hierarchy: Primary sensory → higher order sensory unimodal (perception) → PAA → AAA (multimodal) → premotor → primary motor → SC * Basal ganglia selects the best plan → if i don't have basal ganglia i won't settle on one motor plan (doesn't initiate movement) * Cerebellum → when i find that the plan is not the best while doing the movement → it replans the movement & changes ➢ Ex: carrying something & discovering its heavy → sensory info send copy to cerebellum (so always copy of movement & actual moving) → comparing btwn the intended (planned) & executed movement → if no error → complete/ if not & there is error → replans * Both are also involved in learning from older movements & plans ● AAA: responsible for conceptual cognitive functions & planning motor actions & sends info to progressively lower order motor cortical areas ● Actual processing of the motor response output is executed via the premotor & primary motor areas. ● Therefore: primary sensory areas are the initial sites of cortical processing of sensory information, while the primary motor areas are the final sites for the cortical processing of motor commands. Recap from old lecture: ● Example: ○ If i want to take the cup I need to know where it is in space & what it is (color, shape, & texture etc.), ○ So using my usual sensory stimulation I will see where the cup is then I will measure the distance between me & the cup, this is the work of the posterior association cortex (where & what) ○ If I have left sided neglect, when I hold it with my left hand I won’t be able to recognize it. PAA will then send the information through the upper and lower stream to the anterior association cortex (blue arrows in picture B). ■ Dorsal stream will carry information about the location (where) ■ Ventral stream will carry information about the characteristics (what) ○ Once I measure the distance I will start planning whether I will reach for it or not, is it appropriate for me to take it or not, this is my anterior association cortex, which received info from the “where” & “what” neurons of PAA ○ After I have all that info & made my decision I will move to motor plans. ○ I have different strategies to execute the movement (reaching from the side or turning around & getting the cup etc.). ○ The premotor cortex is providing me with the blueprint or the plan (what muscle to contract or relax etc.) for the movement, both supplementary and lateral. So the premotor is now at a higher level, it is telling me something more elaborate. ■ If the sequence is complex the supplementary would be more involved ○ Then I will execute the action, primary motor cortex ● Throughout this whole process I have ○ Spatial recognition ○ Nature of the object recognition ○ Decision whether to act or not ○ How to act ○ And to act & execute ● The whole thing is about a plan that has a purpose based on a decision (decided by the reticular formation, if I am not thirsty for example reticular formation would shut down the whole process) ● When we were talking about the sensory hierarchy, we started with the primary sensory complex (most simplistic) to higher order sensory cortex (perception) -> higher cognitive function -> premotor -> primary motor ○ So when it comes to sensory I start with simple then move to a more complex stage, but with motor I start with more complex & make my way down to a simple level. Motor function of the basal ganglia: (tmw lecture) ● Enables practised motor acts ● Modulates motor programs stored in the motor cortex & gates the initiation of voluntary movement ● Normally active in suppressing inappropriate motor programs ● Activation of the direct pathway temporarily releases one motor program from inhibition ● Voluntary movement are not initiated in basal ganglia but in cortex ○ Enables proper motor program → direct pathway ○ Inhibits competing motor programs → indirect pathway Cerebellum & the motor cortex: ● Stores learned sequences of movements. ● Participates in fine tuning & coordination of movements produced elsewhere in the brain. ● Integrate sensory & motor info to produce movements so fluid & harmonious that we are not aware of them Control of motor signals: Feed forward: ● ● ● ● Movement that are not correctable during movement aka ballistic Sensorimotor loop is not completed by sensory feedback (open loop) Short delay in the sensorimotor (100ms, ex: saccades of the eyes) Disadvantage: any error in planning or execution can not be corrected (ex: throwing something) cannot adjust Feedback: ● Action (movement) is corrected as it is generated (close loop) ● Senses error between actual & desired position ● The sensory information is fed to a comparator for continuous adjustment ● Can adjust as i execute it (ex lifting a heavy box) Take home messages ● Prefrontal cortex: Selects appropriate behaviour & target (am i motivated?) ● Premotor cortex: Combines info for needed movement programming ● Supplementary motor area: Assembles sequences of movements (complex) ● Primary motor cortex: Executes voluntary movement (by sending projection to sc) receive inputs from cerebellum ○ Adds control of force and direction ● Basal Ganglia: Integrates info from secondary areas and somatosensory cortex creating smooth and integrated movement (any time i have success with one motor plan i learn that its good) ○ Also involved in learning motor sequences. cognitive component too ● Cerebellum: Maintains balance and refines movements( as uy execute them) . ○ Also involved in learning motor skills. Cognitive component too *Basal ganglia & cerebellum are motor & cognitive